Origins of Extreme Liquid Repellency on Structured, Flat, and Lubricated Hydrophobic Surfaces

There are currently three main classes of liquid-repellent surfaces: micro- or nanostructured superhydrophobic surfaces, flat surfaces grafted with “liquidlike” polymer brushes, and lubricated surfaces. Despite recent progress, the mechanistic explanation for the differences in droplet behavior on such surfaces is still under debate. Here, we measure the dissipative force acting on a droplet moving on representatives of these surfaces at different velocities $U=0.01–1\mathrm{mm}/\mathrm{s}$ using a cantilever force sensor with submicronewton accuracy and correlate it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal ($<0.1\mathrm{s}$) resolutions. We find that the dissipative force—due to very different physical mechanisms at the contact line—is independent of velocity on superhydrophobic surfaces but depends nonlinearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform future work on liquid-repellent surfaces and enable their rational design.

Figure 1

Schematics of liquid-repellent surfaces. (a) Structured SH surfaces. (b) Flat surfaces grafted with polymer brushes, dubbed SOCAL surfaces. (c) Structured (left) or flat (right) lubricated surfaces. The droplet is shown with a lubricant cloaking layer, which is typical for low-surface-tension lubricants and higher-tension droplets (Supplemental Fig. S1 [17]).

Figure 2

(a) Characteristic force curves for a water droplet moving on the three surfaces measured using a cantilever force sensor. The motor (to move the substrate) was started at time $t=0\mathrm{s}$ and stopped at $t\approx 50\mathrm{s}$. (b) $F_d$ for $1\mu \mathrm{l}$ water droplets moving at speeds of $U=0.01–1\mathrm{mm}/\mathrm{s}$ on superhydrophobic (hexagonal array of micropillars with diameter $d=16\mu \mathrm{m}$, pitch $p=50\mu \mathrm{m}$, and height $h_p=30\mu \mathrm{m}$), SOCAL, and lubricated surfaces (filled circles, filled squares, and empty circles, respectively). $U$ of droplets tilted at different ${\theta }_{\text{tilt}}=25°–90°$ on the same SOCAL surface and hence subjected to different $F_d=W\sin {\theta }_{\text{tilt}}$ are shown on the same plot (empty squares). $\mathrm{\Delta }{F}_d<0.2\mu \mathrm{N}$ for three repeats, unless otherwise indicated by error bars. See Supplemental Sec. S2 [17] for details on the sample preparation.

Figure 3

Droplet motion on surfaces with ${\theta }_{\text{tilt}}=5°$ and 15°. Depending on whether a droplet is moving with a constant speed or constant acceleration, the displacement $x$ varies linearly or quadratically with $t$, respectively.

Figure 4

Reflection interference contrast microscopy is used to visualize (a) the intercalated air film on a SH surface, (b) the contact line on a SOCAL surface, and (c) the intercalated lubricant film on lubricated surfaces. Scale bars are $100\mu \mathrm{m}$ for (a1), (b1), and (c1), $20\mu \mathrm{m}$ for (a2), (a3), and (c2), and $30\mu \mathrm{m}$ for (b2). The dissipative forces $F_d$ are well described by Eqs. (2, 3, 4). Plots in (a) and (b) are generated from a much larger data set (Supplemental Figs. S5 and S7 [17]). For (b) and (c), the errors $\mathrm{\Delta }{F}_d/2a\gamma$ are ${10}^{-2}$ and ${10}^{-3}$, respectively.